The present invention relates to a prosthetic interbody spacer and, in particular, to an implant for use in replacing diseased and damaged intervertebral discs and fusing vertebrae.
Human intervertebral discs generally serve two functions, they cushion and allow movement between pairs of vertebrae. Cushioning is performed by a gel-like material, which forms the inner part of the discs. A fibrous outer layer surrounds the inner gel-like material. Discs are subjected to a great many strains and stresses and are known to deteriorate with age, injury, or disease. When discs are damaged or diseased, the mobility of the subject is often impaired and great pain may result from movement. Damaged discs may also place pressure on the spinal column, causing pain.
In order to relieve some of the pain associated with disc injury and disease, it has been known to remove the diseased or damaged disc and fuse together adjoining vertebrae. Fusion is often desirable because it serves to fix the vertebral bodies together to prevent movement and maintain the space originally occupied by the intervertebral disc. The loss of mobility associated with this is often acceptable to the patient.
One method of replacing damaged or diseased discs it to insert bone graft material and utilize metal plates and screws to hold the graft material in place until healed. Healing can take a long time period, requiring inactivity for as long as six months. Problems often arise as a result of his type of vertebral fusion, such as slippage, rejection, or failure of the implant.
Discs may also be replaced using a manufactured implant, made of a synthetic material that is biologically compatible with the body and the vertebrae. These prosthetic vertebral interbody spacing devices have been used to both replace discs and vertebrae. They have taken on a number of forms including plug-like, disc-like, and other structures.
Plug-like spacers have taken on a variety of compositions and geometries, ranging from simple blocks of material to carefully shaped implants. An example of a plug-like intervertebral spacer is shown in U.S. Pat. No. 5,263,953 to Bagby, which discloses a coil-type implant which is inserted posteriorly. Plug-like implants often require that bone matter be removed, such as through drilling, in order to allow for placement of the plugs. See, e.g., U.S. Pat. No. 5,015,247 to Michelson; U.S. Pat. No. 4,834,757 to Brantigan. The device disclosed in Bagby includes threads which help to separate the viable bone of the vertebrae from the graft bone, placed in an implant cavity which is drilled into the vertebrae.
Another group of implants are more annular or disc-like in shape. These implants also often require that bone matter be removed in order to provide proper seating on adjacent vertebrae. An example of a disc-like implant is shown in U.S. Pat. No. 5,192,327 to Brantigan et al., which teaches using flat rigid generally oval-shaped discs with ridges to bite into the vertebrae and having slots and hollow areas for packing bone graft material. The Brantigan et al. device incorporates rings which may be stacked and used to replace disc and vertebrae space and requires that adjacent vertebral endplates be prepared by flattening with a burr drill in order to provide a face which is compatible with the shape of the prosthetic.
Disc-like implants are often designed with load-bearing in mind. U.S. Pat. No. 5,397,365 to Kozak et al., the disclosure of which is incorporated herein by reference in its entirety, teaches the desirability of designing the spacer such that the load is transmitted between adjacent vertebrae at the strongest part of the vertebral body, thus avoiding cavitation of the device into the surrounding vertebral endplates with collapse of the intradiscal space and possible damage to the vertebrae itself.
Discs may be replaced using both posterior and anterior techniques. Plug-like devices are often inserted posteriorly. Larger disc-like devices may typically only be inserted anteriorly due to their larger size. The present invention relates to a device which is inserted annularly.
With disc replacement, it is desirable to provide a design which promotes bone ingrowth, thus promoting stability. Avoiding retropulsion or dislodgement is also desirable. In addition, it is advantageous to provide a properly sized and shaped implant which impacts the hard cortex rim bone of the vertebrae and spans the softer interior cancellous bone material of the vertebrae. It is also desirable to reduce the amount of preparation required to the endplates of the adjoining vertebrae and to reduce the amount of bone material removed prior to insertion of the replacement disc.
There is, therefore, a need for an improved spinal disc implant which is both readily utilized in a surgical procedure and advantageously meets the above and other objectives without undesirable side effects.
Accordingly, the present invention is directed to an interbody spacer. The spacer preferably includes an annular base portion having an anterior end, a posterior end, and two side portions together forming a continuous outer wall. The spacer also includes an upper and a lower face portion extending around the outer wall. A plurality of teeth preferably extend outwardly from the upper and lower face portions. The teeth are slanted in the longitudinal direction toward the anterior end and have a generally regular V-shaped configuration in the transverse direction.
In a preferred embodiment, the base portion is generally convex in the transverse direction. The base portion may also be wedge-shaped and concave in the longitudinal direction.
The teeth are preferably configured and dimensioned in the shape of four-sided pyramids to facilitate penetration of the end faces of vertebrae. The teeth may substantially cover the entire upper and lower faces of the base portion and are preferably aligned longitudinally and transversely in a plurality of rows. The teeth may be substantially symmetrical in the transverse direction.
The annular base portion may include an inner wall having at least one groove formed at least around a portion thereof and is wedge-shaped in order to aid in restoring normal anatomical posture. For this purpose, the base portion has a greater height at the anterior end than at the posterior end.
In a preferred embodiment, the base portion is generally oval. The oval may include at least one flattened end. The annular base portion is preferably shaped to substantially conform to an endplate of an adjoining vertebrae and is configured and dimensioned to receive a bone grafting material.
The outer wall of the base portion may include at least one groove defined therein for joining with an insertion device. The outer wall may also include at least one aperture extending therethrough for joining with an insertion device.
The implant may replace the entire disc or may be inserted into the fibrous annulus of an intervertebral disc.